Connected: An Internet Encyclopedia
RFC 821

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Network Working Group
Request for Comments: 821
Replaces: RFC 788, 780, 772

J. Postel
ISI
August 1982

### SIMPLE MAIL TRANSFER PROTOCOL

Jonathan B. Postel

August 1982

Information Sciences Institute
University of Southern California
4676 Admiralty Way
Marina del Rey, California 90291

(213) 822-1511

### Table of Contents

1. INTRODUCTION
2. THE SMTP MODEL
3. THE SMTP PROCEDURES

3.1. MAIL
3.2. FORWARDING
3.3. VERIFYING AND EXPANDING
3.4. SENDING AND MAILING
3.5. OPENING AND CLOSING
3.6. RELAYING
3.7. DOMAINS
3.8. CHANGING ROLES

4. THE SMTP SPECIFICATIONS

4.1. SMTP COMMANDS

4.1.1. COMMAND SEMANTICS
4.1.2. COMMAND SYNTAX

4.2. SMTP REPLIES

4.2.1. REPLY CODES BY FUNCTION GROUPS
4.2.2. NUMERIC ORDER LIST OF REPLY CODES

4.3. SEQUENCING OF COMMANDS AND REPLIES
4.4. STATE DIAGRAMS
4.5. DETAILS

4.5.1. MINIMUM IMPLEMENTATION
4.5.2. TRANSPARENCY
4.5.3. SIZES

APPENDIX A: TCP Transport service
APPENDIX B: NCP Transport service
APPENDIX C: NITS
APPENDIX D: X.25 Transport service
APPENDIX E: Theory of Reply Codes
APPENDIX F: Scenarios

A Typical SMTP Transaction Scenario
Aborted SMTP Transaction Scenario
Relayed Mail Scenario
Verifying and Sending Scenario
Sending and Mailing Scenarios
Mailing List Scenario
Forwarding Scenarios
Too Many Recipients Scenario

GLOSSARY
REFERENCES

Original text document

Next: 1. INTRODUCTION

Connected: An Internet Encyclopedia
RFC 821